In an impact-driven tool configured so that a piston having a large-diameter portion is slidably fitted into a cylinder, an upper chamber is provided above the large-diameter portion of the piston, a lower chamber is provided below the large-diameter portion, the piston is raised by supplying a pressure oil into the lower chamber, a high-pressure gas in a gas chamber formed above the piston is compressed during the rising process to store the energy, and the piston is lowered by the energy derived from expansion of the above-described gas to strike the upper end of a chisel located below the piston, a switching valve is actuated in conjunction with the upward and downward movement of the piston, and the upward and downward movement of the piston is controlled by the switching valve.
Switching valves, which are employed for such an impact-driven tool, include a spool type in which the valve body is in the form of a round shaft, an annular groove is formed in the outer circumference of the valve body, the annular groove is displaced in the axial direction by the upward and downward movement of the valve body, and the flow channels of a hydraulic oil are thereby switched, as disclosed in Patent Literature 1, and a cylindrical type in which a hydraulic oil flows thereinside, as disclosed in Patent Literature 2.